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Second hand bike. GTZR3000

Shimano 105 9 speedshifter
9 speed rear group shimano
9 speed tiagra rear derailleur

The bike runs fine on large front ring, shifting is perfect.

When I change for an incline to the smaller front ring and apply pedal presure the chain comes straignt off. On a stand without pressure resistance on the drive train, everything moves perfectly. Back on the road and again and pedal pressure sends the chain off.

Can anyone advise if they are aware of potential reason why?

  • Welcome to Bicycles @Iain. The first reason why is frame flex. What rear gears does it occur in - all, or just when the chain is on the biggest rear sprockets? BTW we recommend that new members take the tour to make best use of the site. – andy256 Jan 29 '17 at 21:54
  • Your title seems to have become garbled. Could you edit the second sentence of it, please? I've no idea what it's supposed to say. – David Richerby Jan 30 '17 at 9:35
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    Likely your chain is a few links too long. – Daniel R Hicks Jan 30 '17 at 13:26
  • Any idea what the mileage is on the chain? On the cassette/block? and on the chainrings? Can you try using a chain checker or an inch-ruler to see if the chain is elongated with use? – Criggie Feb 2 '17 at 7:16
  • Oh - can you clarify how the chain falls off? Does it always fall to the left (toward the frame) or to the right (toward the big ring/pedal) ? How flexy is the frame? – Criggie Feb 2 '17 at 7:17
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I could be caused because the chain is not running a straight line from the rear cogs to the front chain rings. If you put the bike on the stand and follow the chain from the top of the cogs to the chainrings. You will see if it follows a straight line. It might be slight off so that it won't jump off when on the stand but it does when there is pressure.

Another reason could the rear cogs wable. If it so, you will need to change the cogs.

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    Thank you, rear cogs are set well with no wobble. Seems straight, another post said it could possibly be due to the chain having too many links, is this a point that could also affect? – Iainlockhart Feb 1 '17 at 20:05
  • Better question, what is the optimum number of links on a chain for a 9 speed running on a 53-39 front crank? – Iainlockhart Feb 1 '17 at 20:08
  • If you goggle it, you'll find out there are few ways to do it. You should check sheldon browns or park tool websites. – Baratier ErebusDuHalm Feb 2 '17 at 4:01
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This sounds like the small ring was used a lot more than the large ring and the chain is jumping off the worn out teeth under pressure. compare the tooth profile of the two chainrings and see if they look different.

enter image description here chainring on the left is clearly worn

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